1. Field of the Invention
The present invention relates to directional couplers, and more particularly, to a directional coupler for use in a mobile communication apparatus.
2. Description of the Related Art
FIG. 4 shows a conventional directional coupler. In FIG. 4, a directional coupler 20 is formed of a first microstripline 21 serving as a first distributed-constant line and a second microstripline 22 serving as a second distributed-constant line disposed substantially in parallel to each other, coupled with each other, and formed spirally in a substantially quadrangular shape with the first microstripline 21 being disposed inside of the second microstripline 22. The first microstripline 21 is connected to an input electrode 23 at one end thereof and to an output electrode 25 at the other end thereof. The second microstripline 22 is connected to an output electrode 24 at one end thereof and to an isolation electrode 26 at the other end thereof. The space g4 between the first and second microstriplines 21 and 22 disposed adjacently with the same number of turns is set narrower than the space d4 between the adjacent turns of the pair of first and second microstriplines 21 and 22. The lengths of the first and second microstriplines 21 and 22 are set substantially equal to one fourth the wavelength at the target frequency.
When a signal is input to the input electrode 23 with a terminating resistor (not shown) being connected to the isolation electrode 26 in the directional coupler 20 configured as described above, two signals with a phase difference of approximately 90 degrees are obtained from the output electrodes 24 and 25 at substantially the same amplitude level.
In the conventional case, however, the second microstripline 22 is longer than the first microstripline 21 by the lengths of several corners. Therefore, the phase difference between the two outputs of the directional coupler 20 shifts from the ideal state, namely, 90 degrees.
Accordingly, it is an object of the present invention to provide a directional coupler in which a shift of the phase difference between two outputs from 90 degrees can be reduced.
The foregoing object is achieved according to the present invention through the provision of a directional coupler in which first and second distributed-constant lines disposed substantially in parallel to each other and coupled with each other are formed spirally with the first distributed-constant line being disposed inside of the second distributed-constant line, wherein the space between the first and second distributed-constant lines disposed adjacently with the same number of turns is set wider than the space between the adjacent turns of the pair of first and second distributed-constant lines.
The directional coupler according to the present invention may also be provided with only a portion where the space between the first and second distributed-constant lines disposed adjacently with the same number of turns is set wider than the space between the adjacent turns of the pair of first and second distributed-constant lines.
With such a configuration, a deviation from 90 degrees of the phase difference between two outputs can be reduced in a directional coupler according to the present invention.
According to a directional coupler of the present invention, since the first and second distributed-constant lines disposed substantially in parallel to each other and coupled with each other are formed with the first distributed-constant line being disposed inside of the second distributed-constant line; and the space between the first and second microstriplines disposed adjacently with the same number of turns is made at least partially wider than the space between the adjacent turns of the pair of first and second microstriplines; a shift of the phase difference between the two outputs of the directional coupler from 90 degrees is made small, and the frequency band width of the directional coupler is made wide. In addition, with this structure, the space between the first and second microstriplines disposed adjacently with the same number of turns can be changed in a wide range, and the coupling degree of the directional coupler becomes easier to adjust.